a) Field of the Invention
The present invention relates to a reflecting objective system suited for use as an objective system for microscopes and the similar optical instruments.
b) Description of the prior art:
Unlike the ordinary refraction type objective lens systems, the reflecting objective systems have a common characteristic that they produce no chromatic aberration. Owing to this characteristic, the reflecting objective systems have long since been used as objective systems for telescopes, microscopes and the similar optical instruments. Further, reflecting objective systems are also used in certain cases where it is impossible to obtain materials which are suited for manufacturing lenses of refraction type optical systems to be used at certain wavelengths.
As the conventional reflecting objective systems, there are known the optical systems disclosed by Japanese Patent Kokoku Publication No. Sho 47-12508, Japanese Patent Kokai Publication No. Sho 47-24833, Japanese Patent Kokai Publication No. Sho 59-77403 and Japanese Patent Kokai Publication No. Sho 59-124311.
Out of these optical systems, the reflecting objective system disclosed by Japanese Patent Kokoku Publication No. Sho 47-12508 comprises a combination of two spherical reflecting mirrors and a planar reflecting mirror, and is adapted so as to form an image of object by reflecting light four times on the three reflecting surfaces. It can hardly be said that this reflecting objective system is suited for practical use since optical powers are not distributed adequately among the reflecting surfaces and not only the offaxial aberrations but also the axial aberration is not corrected sufficiently.
Further, each of the reflecting objective systems disclosed by Japanese Patent Kokai Publication No. Sho 47-24833, Japanese Patent Kokai Publication No. Sho 59-77403 and Japanese Patent Kokai Publication No. Sho 59-124311 comprises two reflecting surfaces at least one of which is designed as an aspherical surface, and has the axial aberration corrected relatively favorably by the effect of the aspherical surface. However, the offaxial aberrations are not corrected sufficiently in these reflecting objective systems, thereby making it impossible to obtain large field numbers. These systems are of course usable as objective system when field numbers not so large are allowable for practical use. However, there remains a problem that these reflecting objective systems are not practically usable with ease since it is very difficult to manufacture aspherical surfaces with high precision even when reflecting objective systems have high design performance.